Viscous clutches are used in a wide variety of automotive fan drive applications, among other uses. These clutches employ a relatively thick shear fluid or viscous fluid (typically silicone oil) for the selective transmission of torque between two rotating components. It is possible to engage or disengage the clutch by selectively allowing the shear fluid into and out of a working area of the clutch located between input and output members (e.g., between an input rotor and an output housing). A valve is used to control the flow of the shear fluid in the working area between the input and the output members. Recent clutch designs have been employed that allow the shear fluid to be stored in the rotating input portion of the clutch while the clutch is disengaged, in order to keep kinetic energy available to the shear fluid to allow rapid engagement of the clutch from the off condition. This also allows the clutch have a very low output speed (e.g., fan speed) while in the off position. It has also become common for the clutch to be controlled electrically. This has been done to increase the controllability of the clutch, and to also have the clutch capable of responding to multiple cooling needs in a vehicle. Some of the possible cooling needs are coolant temperature, intake air temperature, air conditioning pressure, and oil temperature.
However, viscous clutches suffer from a problem commonly referred to as “morning sickness”. The problem of morning sickness arises because of the presence of openings or bores that fluidically connect the reservoir and the working chamber. When the clutch is “off”, such as when a vehicle in which the clutch is installed sits unused overnight, the shear fluid can drain back from the reservoir into the working chamber. The drain-back problem is often dependent upon the rotational (or angular) orientation of the clutch when the clutch comes to rest, with gravity tending to induce relatively large volumes of drain back fluid into the working chamber when an opening or bore is rotated so as to be in a lower part of the clutch where that fluid settles. When the vehicle is started, such as a “cold start” the next morning after non-use overnight, the migration or drain-back of the shear fluid into the working chamber can cause significant engagement between the input and output members. For a fan clutch, this can cause relatively high speed fan engagement upon vehicle start-up, which can generate unwanted noise and unwanted cooling effects. Even though the clutch will eventually pump unwanted shear fluid out of the working area to disengage the output member, it would be more desirable to reduce or avoid any time period of clutch engagement due to morning sickness fluid drain-back.
A variety of solutions have been proposed to address the problem of “morning sickness”. Many of those known designs utilize relatively complex structures that make clutch manufacturing and assembly more difficult. Moreover, known morning sickness prevention mechanisms can undesirably increase a size of the clutch in the radial and/or axial direction.
Therefore, it is desired to provide an alternative viscous clutch system that reduces “morning sickness”.